Geological history of the opolszczyzna region

PALAEOZOIC ERA (Devonian – Carboniferous)

The oldest rocks, visible on the surface, located in the southern part of the Opolszczyzna region are the formations dated back to upper Devonian and lower Carboniferous. The largest uncoverings of these rocks are in Opawskie Mountains, but small outcrops can be observed also in the vicinity of Głubczyce (especially in the deeper river valleys), Toszek (Zamkowa Mountain) and Żyrowa (near the fire station, currently covered with soil). These are mainly dark grey mudstones and sandstones (the so-called. "greywacke") composed of a variety of minerals, among which there are feldspars, mica and quartz. Plant fossils, like the carbonized remains of equisetum and fern sterns, or, less frequently, leaves or seeds, are a common encounter. Animal fossils are very rare and badly preserved, majority of them were found in Toszek. On the Czech side of the border however, in the similar natural conditions, nice examples of ammonoidea have been found. These rocks raised from the deep-water oceanic reservoir, lying in a close vicinity to the land. Rivers delivered sand and sludge coming from the destruction of rocks from the land to the sea, where it was settling on the coastal area, on the shelf. Every now and then, the collected material slid on the continental slope, deeper into the ocean as a result of e.g. earthquakes or storms. During that chute the selection of grains in respect of their size was taking place and because of that on the oceanic bottom recurring sequences of sandbank were created: conglomerate – sandstone – mudstone - claystone. Such regular sequences, arising as a result of currents transporting the sediment suspension into the deep, are called Flysch. The most well known Flysch is, of course the one constructing a significant part of the Carpathian mountains, but also the previously described greywacke and mudstones from the Opolszczyzna region.

The external layer of the globe, called lithosphere, is divided into gigantic continental or oceanic plates, continuously shifting on the half-liquid base. As a result, the arrangement of lands and oceans in the geological past was completely different from the present day. In the turn of Devonian and Carboniferous, Opolszczyzna was located in a completely different place – on the southern hemisphere, close to the equator, over 6500 km from of the contemporary location.

About 318 million years ago, in the turn of early and late Carboniferous, the intense orogenesis resulted in drastic changes. Deep seas and oceans of the Silesia vanished, the Palaeozoic deposits (including Devonian and early Carboniferous) have been corrugated, sometimes pulled up into the land and sometimes subjected to slight metamorphosis. On the surface, instead of oceans, mountain ranges developed from these deposits. In the area of the nearby Wałbrzych, in a great mountain valley, gigantic marshes overgrown with thick vegetation developed, which were then transformed into deposits of coal. Similar situation took place in the neighbouring Upper Silesia region, however the vegetation there developed mainly over the sea banks. There are no deposits that can be dated back to the upper Carboniferous and Permian in the region of St. Anne Mountain. Maybe they primarily existed but were damaged by erosion? Or maybe the conditions were not favourable for the permanent rock sedimentation? We do not know. We know that during a long period of the late Carboniferous and Permian, lasting nearly 70 million years, many early Carboniferous rocks in the area of Opolszczyzna were subject to erosion, as the younger Triassic rocks are placed horizontally on the layers of early Carboniferous sandstones that are firmly inclined and cut by erosio., creating the so-called angular inconsistency. Before the Triassic, the mountains mentioned before were also damaged.

MESOZOIC ERA

Triassic

In the Triassic, namely within the time range of 250-200 million years ago, the region of St. Anne Mountain, which today is located at the latitude of 50 0 '30, was significantly displaced to the south (over 3 000. km from today's location), to the vicinity of the Tropic of Cancer (23o), which is the present latitude of the Persian Gulf. For this reason, the climate in Opolszczyzna region used to be hot and dry back then. At the beginning of the Triassic, continental deposits, typical for semi - arid regions, settled in the Opole Silesia region. These were mainly mudstones and sandstones from rivers and shallow but extensive lakes. Rivers and lakes would fill with water during the period of rare, but pouring rainfalls. For the rest of the time the stream channels were empty. We can thus call them temporary reservoirs. Part of the red deposits probably is an example of former dunes, meaning large sand hills, continuously ruffled by desert winds. Such, arising as a result of the wind activity, are called Aeolian. Topography was quite levelled, lowland, without larger mountains because the long-term erosion in the Permian destroyed mountain ranges that arose during the carboniferous orogenetic movements.

In the continental formations of the Lower Triassic the fossils are scarce and they are hardly diversified. What prevails are microscopic fossilized plant pollen and very fine (several milimetres) water shellfish skeletons. In some formations microscopic trails of the extinct amphibians and reptiles can be observed. These animals, when stepping on the humid sediment, left an imprint of their paws which was then covered with another layer of sediment, creating the casting of a trace. Many traces were find in Świętokrzyskie mountains, however in Opolszczyzna region such discoveries are absent, probably due to the absence of quarries that would help to uncover such formations What is the reason for the scarcity of fossil remains in the lower Triassic? As mentioned before, such formations were created in dry semi - arid areas. It is a common knowledge that deserts and semi - arid areas are places where few plants and animals can survive. Additionally only insignificant number of organisms have the opportunity to be transformed into a fossil after its death. Even under favourable conditions of the present day seas, approximately only 1 % of animals living in a given place will turn into a fossil. On land, in lakes that dry out periodically and in rivers, the chances to form a fossil are much smaller, owing to the contact of the animal remains with the atmospheric oxygen, which may lead to complete decomposition, as well as because the destruction of the corpses by atmospheric factors, e.g. sand grains that gradually crush the bones or great temperature differences between the night and day on the desert, which results in drying out, cracking and disintegration of the skeletons. Additionally, it usually takes a lot of time to bury the dead plant or animal with sediment, that is why the majority of them falls victim of the scavengers.

246 million years ago, the worldwide trend of raising the level of the oceans led to the gradual flooding of the previously created lands and to the forming of broad, shallow sea, the so-called German Sea, covering nearly whole area of the today's Poland (without the Carpathian mountains), Germany (without the Alps), the Netherlands, eastern France and England. In Poland it existed for 15 million years. The reservoir was to a significant extent isolated from the Tethys ocean, located on the south, by the great islands. Three narrow and long straits that were vanishing from time to time, were not enough to exchange waters and organisms between the reservoirs, and this is why the fauna of the German sea is characterized by a vast endemism, visible for instance in formations of the quarries in Ligota Dolna and Góra Św. Anny where rock formations consist mainly of species present only in Opolszczyzna region or German sea region. The most important one of the three straits was located in Silesia, in the area of Ostrawa in Czech Republic, from where it ran south, directly towards the ocean. For this reason, in Opole more rocks containing animal remains from the distant south ocean can be found than in Germany or in the region of Świętokrzyskie mountains. In the lowest part the marine Triassic profile, visible in Gogolin, the evidence of earthquakes like e. g. large rock blocks accumulations, separated and transported into the sea by landslides and waves caused by earthquakes (tsunami) can be found. Later, seismic activity significantly decreased.

The worldwide trend of lowering the level of oceans led to the gradual withdrawal of the sea from the region of Opole at the end of the middle Triassic period (about 230 million years ago). Back were the conditions known from the early Triassic – lowlands areas, dry, hot air, temporarily disappearing lakes and periodical rivers. As a result of evaporation, the water in drying lakes was often very salty that is why salts (gypsum), and incidentally, stone salt crystals were formed in the process of precipitation. This is when the coarse complexes of red lake sandstones and lake claystones were created.

During more humid periods however, on the river and lake banks lush vegetation of equisetum, coniferous trees, ferns was developing, which was the staple food for the small animals, which on the other hand were the feed for amphibians and reptiles. Unfortunately, most of these species did not live up to the present day due to unfavourable environmental conditions. However, in several places in the Opolszczyzna region there were short-term favourable conditions encouraging fossilization of skeletons and shells. Several such places are in the neighbourhood of the Lisów village, but the most valuable and famous ones were found in 2000 in Krasiejów near Ozimek. This perfectly preserved and numerous skulls and large fragments of postcranial skeleton became a worldwide sensation. More than 100 large skulls (exceeding 0,5 m) of predatory amphibians, referred to as maze - toothed (because their head and front part of the torso was covered with heavy bone slabs) was found. The body length of amphibians from Krasiejów is estimated for more than 2 meters – not much compared to monstrous dinosaurs from the Jurassic era, but still impressive in comparison with the present day amphibians from Krasiejów, e.g. frogs. Also discovered was the magnificent skeleton of a fish - eating reptile, whose skull measured 70 cm, and estimated body length was 3.5m. The reptile, even though not related to the crocodile, resembled present day gharial living in India. Finally not numerous, but the most valuable findings: the great dinosaur called "Silezaur." It is probably the oldest dinosaur (or its direct ancestor ) in the world.

Cretaceous

The formations dating back to the Jurassic and Lower Cretaceous era are almost nowhere to be found in Opolszczyzna region, nor in Sudety mountains. This applies also to the St. Anne Mountain region. The sea reached St. Anne Mountain at the beginning of late Cretaceous (99 million years ago) and remained here for over ten million years. It can be associated with the largest global raising of the levels of seas and oceans in the history. The maximum level of the ocean was higher than today by about 200 meters, which resulted in drowning almost the whole Europe, including most of the area of Poland. The climate in Opolszczyzna region was significantly warmer than today, and more humid at the same time. In the vicinity of the North Pole the temperature in summer was up to 190 C, and in winter it did not fall below 0 0C. The temperature of surface waters in the seas surrounding the South Pole exceed 200 C. There were no large ice caps on poles, and that is why there was little climatic variability between the distant regions of the Earth.

Opole sea was not very deep, up to 150m. In such a warm and shallow sea the life was thriving. It consisted mainly of organisms with limestone shells which provided the material for creation of lime deposits with. The nearby Sudecka island provided a lot of sand (when land was close) and loam (when the coastline receded along with the flooding of the land). Therefore, initially sands and then marls and limestone was settling. Cretaceous sands of the Senonian age are visible in eastern districts of Opole, on the south of Głubczyce, as well as in St. Anne Mountain region where they were preserved in form of large blocks (70m) immersed in volcanic lava. Marls and limestone on the other hand belonging to the subunits of the Cretaceous- turon and of cognac, were the object of the exploatation in quarries of Opole, and their small amounts are are present in Św. Anny mountain and in the region of Głubczyce (e.g.. Boguchwałów, Nowa Cerekiew).

THE CENOZOIC PERIOD

In Opolszczyzna region there are no rocks from the highest Cretaceous and lower Cenozoic periods (Paleogene – 65 -23 million years), except for Palaeogene basalts. It does not mean that nothing was happening then. The great European plate was at that time pushed by the equally powerful African plate, drawing from the south. Tethys ocean, lying between them was removed, and deposits from the bottom the ocean, as a result of compression between the two plates, acting as jaws of the vice were corrugated, pushed up or pulled inland and often displaced a few hundred kilometers north by the pushing plates. This is how the Carpathian mountains and Alps came into existence (including the Tatra Mountains. Opole Silesia lay on more rigid rocks, far from the removed Tethys ocean, and the changes here were not so drastic. However, the constant pressure of the African plate on the European at the end of Palaeogene caused growing stress inside the Opole rocks. Deep cracks started to form and then along these cracks the parts of the crust were shifting up and down. Such a phenomenon is called a fault. At that time majority of significant faults was created. Sometimes the series of large faults was created in such a way that along them a few dozen meters high block was formed, producing an elevated range, significantly standing out as a long elevation with steep slopes, limited with faults on both sides. An example of such a structure, referred to as tectonic carcass (or horst in German), is e.g. the Chełm Ridge on which St. Anne Mountain is located.

Another consequence of the increasing stresses inside the Earth and creation of deep faults, reaching a few dozen kilometres inside, was liquidation of the parts rocks lying under the crust, and subsequently their outflow along the basalt faults in the form of basalt lava and formation of volcanoes. The majority of volcanoes and lavas in Opole region was created approximately 28-23 million years ago (late Palaeogene, early Neogene).

At the same time hot, humid climate of Paleogene fostered the dissolution of numerous lime rocks from the Triassic in the St. Anne Mountain region and formation of caves inside. A number of small caves was found in the vicinity St. Anne Mountain, the biggest one located in the area of Ligota Górna is Ligocka cave (12 m of length).

In the Neogene (23-1,8 millions years) the deposits were created, first of all, on south of the St. Anne Mountain, in the previously shaped tectonic ditch of Kędzierzyn and near Głubczyce. They were e.g. the formations of shallow sea, periodically drying out, because large deposits of gypsum were then created, extracted in Dzierżysław up to 1974 and are still extracted today on the Czech side of the border.

About 630 000 years ago an ice sheet entered the southern Opolszczyzna (the so-called. Southern Polish Glaciation), gradually moving from Scandinavia. It seems certain that even St. Anne Mountain was covered with thick continental ice sheet. After a long warming and the withdrawal of ice masses, another ice sheet (the so-called Odra Glaciation) appeared 300 000 years ago. At the time maybe only the top of St. Anne Mountain was visible above the surface of ice. During the decay phase of both glaciations great mass of deposit brought from the bottom of the Baltic Sea and Scandinavia, including large rocks, mainly granite, known as glacial erratic was formed. These glacial clays, sands and muds from rivers flowing from the continental ice sheet formed a significant cover of post-glacial deposits, located on the north and south of the Chełm Ridge region. On the very Ridge the erosion removed majority of such formations. The last glaciation (the Northern-Polish Glaciation, approximately 20 000 years ago) did not reach Opolszczyzna, however, fine dust flown from the ice fields of northern Poland was settling there, creating thick rock covers known as loess, characteristic especially for Głubczycki Plateau, but present also St. Anne Mountain. The loess later transformed into extremely fertile soils and this is why the early and farming was well developed there.

* Thanks to the courtesy of Gazeta Wyborcza, Opole, the fragments of text by Robert Niedźwiedzki, published in Gazeta Wyborcza 3 XI and 30 XII 2006 were used in this study. Also the archaeological consultation provided by Dr Andrzej Wiśniewski from the University of Wrocław was used here.